The present invention relates to a brake power servo booster, and in particular to a brake power servo booster of the stroke enlarging type in which the amount of stroke of an output shaft can be enlarged more than that of an input shaft.
Various arrangements of such a stroke enlargement type brake power servo booster have been known. Such a stroke enlargement type brake power servo booster requires an enlarging mechanism for enlarging the amount of stroke of an output shaft relative to that of an input shaft, and a lock mechanism for preventing the stroke enlarged by the enlarging mechanism from decreasing especially in the full load state of the brake power servo booster. In the conventional brake power servo booster of this type, these mechanisms are separately provided and each of them is complicated so that the brake power servo booster is not only large in size but expensive. This type of brake power servo booster is disclosed in U.S. Pat. No. 4,386,554.